Bernard L. Madoff has maintained for years that his decadeslong Ponzi scheme was a one-man show. On Monday, a jury concluded instead that it was a team effort.

Jurors found five former employees of Mr. Madoff guilty of aiding and hiding the fraud in a trial that painted the money manager's Manhattan offices as a hive of illegal activity, where employees cooked up lies and manufactured fake documents to keep afloat a scam that...